1. Field of the Invention
The present invention relates to a method for growing a silicon carbide single crystal, and more particularly, to a method for growing a silicon carbide single crystal by means of molecular beam epitaxy.
2. Description of the Prior Art
Silicon carbide is a material for a semiconductor having a wide forbidden bandgap of 2.2 eV-3.3 eV, at 300.degree. K. Silicon carbide is desirable as a material for a visible short wavelength light emitting device (especially emitting blue), since such a semiconductor made from silicon carbide can become either a p-type or a n-type depending upon whether impurities are added. Furthermore, silicon carbide is exceedingly stable thermally, chemically and mechanically, and has an advantageous characteristic of a strong radiation damage resistance.
On the other hand, semiconductor elements using conventional materials such as silicon are difficult to use under a severe condition, especially at a high temperature, with a high power density capability, or with being irradiated by radiation. Therefore, semiconductor elements using silicon carbide are desirable for use under the above-mentioned severe conditions in a wide variety of applications, for example, for a rectifier, a radiation detector, an ultraviolet detector, a junction and MOS type field effect transistor (FET), various power elements, and for a light emitting diode (LED).
However, a crystal growth technique for a stable supply of large silicon carbide single crystals having high quality on a commercial scale has not yet been established. This is because a silicon carbide crystal is difficult to grow due to its thermal and mechanical stability. Therefore, although silicon carbide is a material for a semiconductor with many advantages and possibilities as is described above, it has been difficult to put to practical use.
The recrystallization method and the epitaxy method are known methods for growing silicon carbide single crystals. As for the recrystallization method, silicon carbide single crystals conventionally have been grown on a laboratory scale using the sublimation recrystallization method (Lely method) at a high temperature of 2,500.degree. C. or more. In this method, SiC powder in a graphite crucible is vaporized by heating in an Ar gas, and then is transferred to a low temperature portion to be recrystallized, thereby obtaining thin plate crystals, most of which are 6H-SiC, with a maximum thickness of about 10 mm in about 6 hours.
However, since these crystals are grown on an abiogenetic core, the shapes are not constant and the sizes are limited. Therefore, crystals with high quality cannot be grown.
Alternately, in the epitaxy method the vapor phase epitaxy, the liquid phase epitaxy and the molecular beam epitaxy are included in the epitaxy. In this method, a silicon carbide crystal obtained in the Lely method is used as a substrate (a seed crystal) on which silicon carbide single crystals are further grown to obtain silicon carbide single crystals that are large enough to produce semiconductor elements.
However, the single crystals obtained in the above methods still are relatively small, and it is difficult to regulate the sizes and the shapes thereof with high accuracy. Furthermore, it is also difficult to regulate the crystal structures, such as 4H-, 6H-, or 3C-modifications, of silicon carbide, and to regulate the impurity concentration.
In order to solve the above-mentioned problems, a modified Lely method has been introduced. In 15 the the modified Lely method, single crystals are grown on a seed crystal in vacuo of about 1.5 torr so as to obtain ingot single crystals of silicon carbide. (See Y. M. Tairov and V. F. Tsvetkov, J. Crystal Growth, 52: 146-150 (1981); G. Ziegler, et al., IEEE, ED-30:277-281 (1983)). In the modified Lely method, silicon carbide single crystals are grown in a graphite crucible using silicon carbide as a source material with the following serving as parameters: (1) the temperature of the seed crystal; (2) the temperature gradient of the seed crystal and the material; and (3) the pressure in growing. Accordingly, silicon carbide single crystals can be grown with regulating the crystal structures and shapes to be grown.
However, the change with time in the sublimation condition of the material, that is, the components of the substance evaporated from the source material, causes a difference in the growing conditions between in the initial stage and in the stage just before finishing. For example, the sublimation of silicon carbide (SiC) generates molecular seeds of SiC.sub.2, Si.sub.2 C, Si and C. At a certain temperature, the seeds of the Si group sublimate first, and when the growth is finished, only carbon (C) may sublimate and grow.
Accordingly, various crystal structures are produced and large silicon carbide single crystals are difficult to obtain.
On the other hand, in the conventional molecular beam epitaxy using a material gas including silicon (Si) and carbon (C), the growth rate is small and only 3C(B)-silicon carbide films can be obtained. The epitaxy using SiC has not been known so far.